1. Technical Field
The present invention relates to network line equalizers having filters for equalizing attenuated transmitted analog signals, such as multiple layer transition (MLT-3) decoded signals, from a network medium such as a 100-BASE-TX Ethernet (IEEE standard 802.3u) transmission medium.
2. Background Art
Local area networks use a network use a network cable or other network media to link nodes (e.g., workstations, routers and switches) to the network. Each local area network architecture uses a media access control (MAC) enabling a network interface device at each network node to share access to the media.
Physical (PHY) layer devices are configured for translating digital packet data received from a MAC across standardized interface, e.g., a media independent interface (MII), into-an analog signal for transmission on the network medium, and reception of analog signal transmitted from a remote node via the network medium. An example is the 100 BASE-TX IEEE standard 802.3u receiver, configured for receiving a 3-level MLT-3 encoded analog signal at a 125 Mb/s data rate.
One problem with transmission of analog signals on the network medium is the attenuation of high-frequency components. For example, FIG. 1A is a diagram illustrating simplified frequency response characteristics f(line) of the network medium. As shown in FIG. 1A, an MLT-3 encoded signal transmitted by the network medium encounters transmission loss in the form of high-frequency attenuation. Hence, the 100-BASE-TX Ethernet (IEEE 802.3u) receiver includes a line equalizer having a high-pass filter, having the frequency response (f(filter)) of FIG. 1B to compensate for the high-frequency attenuation from the network medium. One example of a high-pass filter is a single zero filter.
FIG. 2 is a diagram illustrating a conventional single zero high-pass filter 10. As shown in FIG. 2, the high-pass filter 10 includes an operational amplifier 12, a capacitor 14 having capacitance C., a resister 16 having resistance R. As recognized in the art, the high-pass filter 10 has a transfer function H(s)=S+Z, where Z equals 1/RC. Hence, the high-pass circuit 10 is considered a single zero filter, where S is a complex variable based on frequency.
A disadvantage of the high pass filter 10 is that a high bandwidth operational amplifier 12 is required for implementation. In addition, a direct connection of the high-pass filter 10 within a line equalizer may affect the impedance of the transmission line (i.e., the network medium), since the capacitor 14 and resistor 16 are in parallel with the transmission line's termination resistance. In addition, the connection of the capacitor 14 is between the two nodes (V.sub.IN and V.sub.O), neither of which is a ground or a supply node. Hence, the high-pass filter 10 is extremely difficult to implement using CMOS technology, since a CMOS capacitor cannot be connected between two arbitrary nodes.
A single zero equalizer may not always provide the optimum compensation for the line response f(line). For example, FIG. 3 is a diagram illustrating an alternate characterization of the frequency response characteristics f'(line) of the network medium. As shown in FIG. 3, the frequency response f' includes a linear region 10 between points C and D, and a nonlinear lower frequency region 18 between points A and B. Although a single zero equalizer may compensate adequately in the linear region 10, the single zero equalizer may not compensate as well within the range 18 within the lower range of frequencies. Hence, some other function is needed to compensate for the attenuation in the transmission medium according to the function f'(line). More precise compensation can be obtained over a single zero filter with the addition of the single zero, single pole filter.
A single, single pole filter operates according to the transfer function H=(s+z)/(s+p), where z is a zero and p is a pole located higher than zero in the frequency range. Single zero, single pole filters are typically implemented using an operational amplifier or a switched capacitor filter, which significantly increases the complexity of the design.
More precise filtering of received network signal may be performed using an equalizer having filters operating according to biquadratic functions, for example H=(S+Z1)(S+Z2)/(S+P1)(S+P.sub.2). The function "H" is described as a biquadratic function, since the numerator and denominator consist of quadratic equations. Biquadratic equalizers are typically implemented using either a single operational amplifier and a significant number of passive components, or several transconductance amplifiers in a feedback loop, as shown in FIG. 4. Use of a large number of passive components is discouraged in CMOS design, due to the large area, poor parameter control, and unavailability. Feedback loops, however, introduce stability problems that need to be simulated carefully to insure that the circuit is stable throughout the entire frequency range.